FIG. 12 shows an example of the construction of a stage apparatus and its control system for positioning an object in a semiconductor exposure apparatus, a machine tool or the like (for example, see Japanese Patent No. 3176766).
In FIG. 12, a table top plate 101 is supported with guides 102 movably in leftward and rightward directions of the surface of the drawing. Four movable magnets 105 are arranged on the table top plate 101 such that the polarities of the magnets are alternately changed with respect to a vertical direction of the surface of the drawing. The movable magnets 105 and coils 104 arrayed at equal intervals in a moving direction of the table top plate 101 form a liner motor. The position information of the table top plate 101 in the moving direction is inputted by a position detector 106 and a counter unit 107 into a servo controller 108 and a phase selection controller 109. To solve a positional difference, which is a value obtained by subtracting a position measurement value of the table top plate 101 from a position command to the table top plate 101, the servo controller 108 calculates a control command to the linear motor.
The phase selection controller 109 calculates a rectification value to the respective linear motor coils 104 based on the relative positional relation between the movable magnets 105 and the linear motor coils 104 obtained from the position measurement value of the table top plate 101. A multiplier 119 multiplies the above control command to the linear motor by the above rectification value via a filter circuit 113 and sends the command to the respective current amplifiers 111, to appropriately cause a thrust with the linear motor coils 104, thereby attains position control of the table top plate 101.
A thrust constant of the linear motor is represented with a thrust [N] to a current 1.0 [A] flowing through the coil, as N/A. It is ideal that the thrust constant is constant regardless of the relational position between the coils and the movable magnets, but actually, it is known that the thrust constant varies within a predetermined amplitude. The variation of the thrust constant is called a “thrust ripple”. For example, in a case where a 1% thrust ripple exists, the thrust constant, normally 1[N], varies within a range from 0.99 to 1.0 [N] depending on the position.
One of indices representing the performance of a semiconductor exposure apparatus is throughput as the number of processed wafers per hour. To realize high throughput, it is necessary to increase the speed and acceleration upon driving of the stage. As the influence of the thrust ripple is multiplication to the thrust, the error of thrust is increased as the thrust of the linear motor is increased.
Accordingly, when a high thrust is required upon acceleration or deceleration of the stage, the positional deviation of the stage is increased since the actual thrust is extremely shifted from a desired value, and as a result, stabilization time required before the positional deviation of the stage is converged to a predetermined value and exposure becomes possible is increased. The increase in the stabilization time reduces the throughput. Further, in recent years, the use of a scan type exposure apparatus which performs exposure while scanning a reticle stage and a wafer stage in synchronization with each other is a main stream. The wafer is exposed when the reticle stage and the wafer stage are scanned at a constant speed. When the stage is scanned at a constant speed, a high linear motor thrust as in the case of acceleration/deceleration is not required, however, the value of the control command to the linear motor is not zero due to friction resistance at guide members and other disturbance, and the influence of thrust ripple also appears. The influence of thrust ripple upon constant-speed scanning of the stage causes the positional deviation of the stage which becomes a factor of degradation of exposure accuracy. That is, the scan type exposure apparatus is influenced by the thrust ripple upon acceleration/deceleration of stage scanning and upon constant speed stage scanning.
As thrust ripple correction, a method of previously measuring a thrust ripple, then generating a correction table and a correction function based on a stage position, and performing thrust ripple correction upon driving, has been performed. Further, a method for measuring a thrust ripple and generating a correction table and a correction function has been proposed (see Japanese Patent No. 3176766 and Japanese Patent Application Laid-Open No. 7-161798).
In the publication Japanese Patent Application Laid-Open No. 7-161798, a current value of a linear motor used upon actual stage driving is recorded, and a ripple correction table with respect to stage position is generated based on the value, and ripple correction is performed upon exposure driving. Further, upon constant speed driving, as a thrust ripple cannot be observed since the current control command to the linear motor is approximately zero, an external force is applied to produce a current flow through the linear motor. That is, it is necessary to accurately obtain the external force for accurate calculation of thrust ripple. However, there are other external forces to the stage than the external force applied for ripple correction, e.g., a force of the stage's own weight by inclination of the surface of a stage base, a spring force with a cable, air piping to the stage or the like, and the external force cannot be accurately obtained. Even with the method disclosed in the above Japanese Patent Application Laid-Open No. 7-161798, the thrust ripple cannot be accurately obtained. Accordingly, desired stage performance cannot always be attained by thrust ripple correction.
In Japanese Patent Application Laid-open No. 2001-175332, a correction function is generated from data upon actual stage driving. More specifically, parameters are set respectively for a thrust ripple due to occurrence of a driving force for a stage driving amplifier, a thrust ripple due to viscosity of the linear motor, and a thrust ripple having a constant amplitude characteristic of the linear motor. The parameters are estimated by an adaptation mechanism. The above three types of items include a thrust ripple other than the thrust ripple in real meaning as variation of thrust constant dependent on the stage position. In the method using data upon actual stage driving, it is necessary to obtain a thrust ripple from a control command including all the disturbances. Accordingly, the number of estimation parameters is increased, and it is difficult to accurately obtain a thrust ripple.
As described above, it is desired to accurately measure a pure thrust ripple for generation of a thrust ripple correction table.
Further, generally, to prevent breakage of the stage, the linear motor and the current driver, in calculation by the control system, a command value to the current driver is limited with an upper value and a lower value. When the command to the current driver before correction is almost the limit value and a correction regarding the thrust ripple is performed on the command to the current driver, the command to the current driver exceeds the limit value by multiplication by a correction value. A thrust ripple correction method for reliable convergence of command to the current driver between the limitation values is needed.
Further, it is convenient for the sake of accuracy that the measurement of thrust ripple is performed in a status where the linear motor is incorporated in the exposure apparatus. Accordingly, a structure to perform thrust ripple measurement in a status where the linear motor is incorporated in the apparatus is desired.
Further, a linear motor having the simplest structure using one stator and one movable element is widely used. Also, a thrust ripple measurement method with a simple structure applicable to this linear motor is desired.
Further, for reduction of exposure apparatus assembly time, a linear-motor thrust ripple correction table may be obtained before the linear motor is incorporated in the exposure apparatus. Further, a method for measuring a thrust ripple before the motor is incorporated in the exposure is needed.
Further, recently, it is conceivable that the number of coils used in an exposure apparatus is increased from conventional several tens to several hundreds due to increase in a stage stroke by use of large wafer diameter (300 mm), adoption of a twin stage system having two stages in one exposure apparatus, or the like.
Further, upon checking trouble of coils or breakage of wirings to the coils, it is not efficient to check the coils one by one. Accordingly, a method for efficiently checking the coils and coil wirings is needed.